1. Field of the Invention
The present invention relates to an optical repeating system that comprises an uplink optical transmission line, a downlink optical transmission line, an optical transmitter and an optical receiver for transferring a main signal through the uplink or downlink optical transmission line, and a plurality of optical amplifying repeaters for amplifying and repeating the main signal with optical amplifiers at intermediate positions on the optical transmission lines, and to an optical amplifying repeater control method for controlling the optical amplifying repeaters.
2. Description of Related Art
FIG. 4 is a block diagram showing a configuration of a conventional optical repeating system disclosed in JP2716882B2. In FIG. 4, the reference numeral 1 designates an optical transmitter for transmitting a main signal conveying information and a first sub-signal bearing a command for an optical amplifying repeater 3; 2 designates an optical fiber constituting an optical transmission line for interconnecting the optical transmitter 1, optical amplifying repeaters 3 and an optical receiver 4; 3 designates an optical amplifying repeater located at an intermediate position on the optical fiber 2 for not only amplifying and transmitting the main signal with an optical amplifier, but also for superimposing the supervisory information obtained from the command conveyed by the first sub-signal on the main signal as a second sub-signal, and 4 designates an optical receiver for receiving the main signal and the like.
FIG. 5 is a block diagram showing a configuration of the optical amplifying repeater as shown in FIG. 4. In the optical amplifying repeater 3 as shown in FIG. 5, the reference numeral 101 designates a coupler for splitting the input optical signal; 102 designates an optical amplifier comprising an isolator 111, a pumping laser diode 112, a multiplexing filter 113, an erbium (Er) doped optical fiber 114, and an isolator 115; and 103 designates a coupler for splitting an optical signal output from the optical amplifier 102, and for outputting its first part as the output optical signal.
In FIG. 5, the reference numeral 121 designates a photoelectric converter for converting an optical signal to an electrical signal; 122 designates an amplifier for amplifying the electrical signal; 123 designates a low-pass filter for filtering the sub-signal; 124 designates an incoming call identification circuit for making a decision as to whether the first sub-signal contains an operation command signal addressed to the present repeater; 125 designates a controller for actuating an encoder 131 and a modulator 132 in response to the operation command signal addressed to the present repeater; 131 designates the encoder for encoding intra-repeater information such as the power level of the output optical signal of the optical amplifying repeater 3, the amplification factor of the optical amplifier 102, the driving current level and temperature of the pumping laser diode 112; and 132 designates the modulator for modulating the driving current to be supplied from the pumping laser diode driver 133 to the pumping laser diode 112 by a supervisory signal including the intra-repeater information after encoding. The reference numeral 134 designates a temperature controller for controlling the temperature of the pumping laser diode 112; 141 designates a photoelectric converter for converting an optical signal to an electrical signal; and 142 designates an amplifier for amplifying the electrical signal.
FIG. 6 is a block diagram showing a configuration of the optical transmitter 1 in FIG. 4. In the optical transmitter 1 of FIG. 6, the reference numeral 201 designates a main signal circuit for outputting the main signal used for information transmission; 202 designates a sub-signal circuit for outputting the first sub-signal containing the operation command that specifies the optical amplifying repeater; 203 designates a modulator for superimposing the first sub-signal on the main signal in a prescribed modulating scheme; 204 designates a driver supplied with the main signal on which the first sub-signal is superimposed for driving a semiconductor laser 205; and 205 designates the semiconductor laser for supplying the optical fiber 2 with the optical signal corresponding to the applied electrical signal.
FIG. 7 is a block diagram showing a configuration of the optical receiver in FIG. 4. In the optical receiver 4 of FIG. 7, the reference numeral 301 designates a photoelectric converter for converting the input optical signal fed from the optical fiber 2 to an electrical signal; 302 designates an amplifier for amplifying the electrical signal; 303 designates a main signal demodulator for demodulating the main signal in the received signal; 304 designates a low-pass filter for filtering the first and second sub-signals in the received signal; and 305 designates a sub-signal demodulator for demodulating the first and second sub-signals in the received signal.
Next, the operation of the conventional optical repeating system will be described.
First, the operation of the optical transmitter 1 will be described. The sub-signal circuit 202 generates the first sub-signal, which includes the operation command specifying one of the optical amplifying repeaters 3 by an address code uniquely assigned to each optical amplifying repeater, in such a manner that its amplitude is smaller and its rate is lower than those of the main signal output from the main signal circuit 201. Then, the modulator 203 supplies the driver 204 with the main signal on which the first sub-signal is superimposed. The output optical signal of the semiconductor laser 205 consists of the modulation signal of the main signal plus the first sub-signal superimposed thereon. The optical transmitter 1 transmits the operation command to the next optical amplifying repeater 3 as the first sub-signal in such a manner that an appropriate time interval is reserved after the first sub-signal including the operation command. The reserved time interval enables the specified optical amplifying repeater 3 to transmit a supervisory signal corresponding to the first sub-signal during the reserved time interval as the second sub-signal.
Next, the operation of the optical amplifying repeater 3 will be described. The coupler 101 splits the input optical signal fed from the input side optical fiber 2. A first part of the split input optical signal is launched into the optical amplifier 102 to be amplified. On the other hand, a second part of the split input optical signal is launched into the photoelectric converter 121 to be converted to the electrical signal. The electrical signal is amplified by the amplifier 122, and then the low-pass filter 123 extracts the first sub-signal with a frequency lower than the frequency of the main signal, and supplies it to the incoming call identification circuit 124. The incoming call identification circuit 124 makes a decision as to whether the optical transmitter 1 sends the operation command to this repeater from the address code contained in the first sub-signal, and notifies the controller 125 of the decision result. When the operation command is addressed to the repeater, the controller 125 actuates the encoder 131 and the modulator 132 to modulate the driving current to be supplied from the pumping laser diode driver 133 to the pumping laser diode 112 by the second sub-signal including the intra-repeater information. Since the driving current to the pumping laser diode 112 is modulated by the second sub-signal, the amplification factor of the optical amplifier 102 is modulated. Thus, the optical signal output from the optical amplifier 102 consists of the main signal and he second sub-signal superimposed thereon. On the other hand, when there is no operation command addressed to the repeater, the controller 125 does not actuate the encoder 131 nor the modulator 132. As a result, the pumping laser diode 112 is driven by a non-modulated driving current.
Finally, the operation of the optical receiver 4 will be described. The input optical signal fed from the optical fiber 2 is converted by the photoelectric converter 301 into an electrical signal which is amplified by the amplifier 302. The amplified electrical signal is supplied to the main signal demodulator 303. In parallel with this, the low-pass filter 304 extracts the first and second sub-signals from the electrical signal, and supplies them to the sub-signal demodulator 305. The sub-signal demodulator 305 demodulates the operation command, which is addressed to the optical amplifying repeater 3, from the first sub-signal transmitted from the optical transmitter 1, and the intra-repeater information from the second sub-signal transmitted from the optical amplifying repeater 3. Thus, the operation state of each optical amplifying repeater 3 can be supervised.
In this way, the optical transmitter 1 selects one of the optical amplifying repeaters 3 one by one, and transmits the operation command by superimposing it on the main signal. Receiving the operation command addressed to it, each optical amplifying repeater 3 superimposes the supervisory information about the repeater on the main signal, and sends it to the optical receiver 4. The optical receiver 4 demodulates the supervisory information sent from the optical amplifying repeaters 3 sequentially.
With the foregoing configuration, the conventional optical repeating system can supervise each of the optical amplifying repeaters 3. However, the conventional optical repeating system has a problem in that it is difficult to adjust the amplification factor of the optical amplifier 102 of the optical amplifying repeater 3, and to stabilize the amplification characteristics of the optical amplifying repeater 3 over the long run.